1. Field of the Invention
The present invention relates generally to an optical scanning device and an imaging apparatus implementing this optical scanning device, and particularly to a multi-beam optical scanning device that simultaneously scans a surface to be scanned such as a photoconductor using a plurality of light beams so as to significantly increase recording speed, this multi-beam optical scanning device being suitable for use in a write system of a recording device such as a digital copier, a laser printer, or a laser facsimile.
2. Description of the Related Art
One known method of increasing the recording speed of an optical scanning device implemented in a write system of a recording device such as a laser printer or a laser facsimile is a method of increasing the rotation speed of a polygon mirror, which functions as a deflector. However, in this method, factors such as the durability of the motor, the noise, the oscillation, and the modulation speed of the semiconductor laser impose limitations on attempts at increasing the recording speed. Thus, a multi-beam optical scanning device that increases recording speed by simultaneously scanning a plurality of light beams to record a plurality of scanning lines at once is being proposed.
Also, technology for forming the scanning lens with resin is being developed in order to realize cost reduction and formation of a particular lens surface.
However, it is known that the change in the curvature radius or the refractive index due to a change in environmental temperature and the like is greater in a resin lens than in a lens made of glass. Thereby, in a multi-beam scanning optical system using the resin lens, the scanning line spacing for the plurality of scanning lines in the sub-scanning direction (referred to as ‘beam pitch’ hereinafter) is varied, causing image degradation such as unevenness in image density.
FIG. 1 is a diagram showing the light path of principal rays in the sub scanning direction in an optical scanning device according to the related art. In this optical scanning device, a glass coupling lens 102 and a glass cylindrical lens 104 are used as a first optical system and a second optical system, respectively. Also, a two-channel LDA (laser diode array) 101 is shown as a light source. Further, resin lenses 161 and 162 (scanning lens) make up a third optical system.
In the following, the change in the beam pitch in response to a change in temperature will be described.
First, a beam pitch P′ on an image surface can be expressed by the following formula:P′=P0×Fcyl/Fcol×βwherein P0 denotes the beam spacing in the sub scanning direction at the light source, Fcol denotes the focal length of the first optical system, Fcyl denotes the focal length of the second optical system, and β denotes the lateral magnification in the sub scanning direction of the third optical system.
Since the change in P0, Fcol, and Fcyl due to temperature change is trivial and can be disregarded, the change in the beam pitch can be described in conjunction with the change in the lateral magnification in the sub scanning direction of the third optical system made up of the resin lenses 161 and 162. That is, the change in the lateral magnification in the sub scanning direction of the third optical system due to temperature change is directly reflected in the change in the beam pitch. The light path indicated by dotted lines in FIG. 1 is the light path when the temperature is increased. That is, the beam pitch increases as a result of an increase in the temperature.
Also, in the optical scanning device, a bundle of rays emitted from a plurality of emission points is normally converted into a bundle of parallel rays via the coupling lens 102 (first optical system), and is formed into a line image that extends along the main scanning direction by the cylindrical lens 104 (second optical system), this being performed in the vicinity of a polygon mirror 105. The polygon mirror 105 deflects each bundle of rays that is emitted via the cylindrical lens 104, and scans this at a substantially isometric speed (constant linear velocity). The scanning lenses 161 and 162 of the third optical system form an image on a surface to be scanned 107 by condensing each bundle of rays deflected and scanned by the polygon mirror 105. However, in order to enhance flexibility in optical design, the bundle of rays emitted from a plurality of emission points is preferably converted into a bundle of diverging rays or convergent rays at the coupling optical system in accordance with the characteristics of the optical systems following the coupling optical system.
In the multi-beam optical scanning device, given that the emission point positions (emission point spacing) change by P1 under the influence of temperature change or a difference in the mounting of the light source device, the beam spacing at the surface to be scanned 107 changes (is degraded) by P, which can be expressed by the formula below:P=P1×m (m: magnification in the sub scanning direction between the light source and the surface to be scanned)
Thus, to obtain stable beam spacing in the sub scanning direction at the surface to be scanned 107, the magnification in the sub scanning direction between the light source and the surface to be scanned 107 is preferably low.
In turn, to lower the magnification in the multi-beam optical scanning device, the bundle of rays emitted from a plurality of emission points is preferably converted into a bundle of diverging rays at the coupling optical system.
When the bundle of rays emitted from a plurality of emission points is converted into a bundle of diverging rays via the coupling optical system, the principal rays of the bundle heading toward the same image height will be parallel (the field angles are equal). Herein, if the scanning lens has functions of imaging parallel rays, the surface to be scanned 107 will be adjusted to a focus point of the bundle of rays (imaging point of the diverging rays). However, the focus point of the bundle of rays and the intersecting point of the principal rays (the intersecting position of the parallel rays) do not meet at the same point. Thus, when the surface to be scanned 107 is adjusted to the focus point, a displacement of dots in the main scanning direction occurs. Also, since the image height at the start of writing and the image height at the end of writing form different angles at the deflection surface of the deflector (e.g. reflection angle of the polygon mirror), the write width (scanning width) of each beam is different and deviation between the beams is created. If the emission points are not spaced out in the main scanning direction (e.g. the emission points of the semiconductor laser array are arranged to be aligned in the sub scanning direction), the displacement of dots in the main scanning direction and the difference in the write width between each of the beams can be prevented. However, the above arrangement is difficult to realize since errors during the mounting of the devices and the like cannot be completely eliminated.
Also, in a case where the scanning lens (third optical system) has a function of imaging diverging rays, each bundle of rays is focused at the surface to be scanned 107. However, if the principal rays of each beam emitted from the different emission points are parallel, the rays intersect before reaching the surface to be scanned 107 (towards the deflector) and thus the image heights will differ at the surface to be scanned 107. Also, since the image height at the start of writing and the image height at the end of writing form different angles at the deflection surface of the deflector (e.g. reflection angle of the polygon mirror), the write width (scanning width) of each beam is different and deviation between the beams is created. Further, the difference in image heights will still exist even when the image height at the start of the writing is adjusted according to the image height at the end of writing.
As described above, when the bundle of rays that has passed through the coupling optical system is converted into diverging rays, degradation of the image such as unevenness in density or distortion of the vertical lines may occur due to the displacement of dots in the main scanning direction or the difference in the write width (scanning width) of each beam.
In recent years, technologies for increasing the density of the image reproduced by the digital copier or the laser printer have been developed, and with this continuing development, the miniaturization of the beam spot diameter on the photoconductor is in demand. However, as mentioned above, a resin lens induces a greater change in the curvature radius or the refractive index due to a change in environmental temperature and the like compared to a glass lens. When a field curvature is generated as a result of the above change in the curvature radius or the refractive index, the beam spot written on the photoconductor will be enlarged, leading to image degradation.
In Japanese Patent Laid-Open Publication No. 8-292388, an optical scanning device developed in response to the above described problems is disclosed. Since the change in the field curvature due to temperature change at the positive lens and the change in the field curvature due to temperature change at the negative lens are inverses (negatives) of each other, the optical scanning device according to this prior art invention is arranged to compensate for the change in field curvature by implementing a scanning resin lens and another resin lens having the inverse power of the scanning lens on the light path between the light source and the light deflector so that the change in field curvature caused by temperature change in the scanning lens is canceled out. However, the resin lenses implemented between the light source and the light deflector have no power in the main scanning direction and thus have no compensation capabilities for the change in field curvature in the main scanning direction caused by the temperature change of the resin scanning lens. Therefore, the enlargement of the beam spot diameter in the main scanning direction cannot be prevented in this prior art invention.
Also, in order to improve the shape of the beam spot in the sub scanning direction, compensation for wave aberration from the standpoint of wave optics needs to be considered as well as compensation for field curvature from the geometric-optical standpoint. In an optical scanning device disclosed in Japanese Patent Laid-Open Publication No. 8-292388, all resin lenses having a negative power are arranged to be plano-concave cylindrical lenses. However, as described in the preferred embodiments of this patent application, the curvature radii will be quite small at around 5 mm or 8 mm according to this prior art invention. Thereby, a higher level of processing precision and/or mounting precision will be required. The problem with this prior art invention is that the temperature compensating function is provided only on one surface of the lens.
Also, in a light beam scanning optical device disclosed in Patent Gazette Publication No. 2804647, the compensation for field curvature in the main scanning direction is realized by a resin lens that has a power opposite (negative) to the power of the scanning resin lens. However, for the sub scanning direction, the change in field curvature is controlled by restricting the mounting position of the scanning lens. With this arrangement, the flexibility in design will also be restricted. Further, this light beam scanning optical device compensates for the field curvature in the sub scanning direction by using a cylindrical resin lens that has a negative power. However, the temperature compensation function is only provided on one surface of the lens. (In the light beam scanning optical device claimed in claim 9 of the Patent Gazette Publication No. 2804647, one side of the lens having the negative power has an axially symmetric aspherical surface and the other side has a cylindrical surface, thereby providing negative powers to both surfaces; however, the axially symmetric aspherical surface is mainly for compensating for the field curvature in the main scanning direction and thus has weak power and the compensation for the field curvature in the sub scanning direction is mainly realized by the other cylindrical surface, which has the stronger power.) Therefore, the curvature radius of the cylindrical surface will be small, and a higher level of processing precision and mounting precision will be required.
Also, in a laser beam scanning optical device disclosed in Japanese Patent Laid-Open Publication No. 10-20225 or a scanning optical device disclosed in Patent Gazette Publication No. 2761723, the misplacement of the image formation position due to temperature change is compensated for by moving a collimator lens and the like towards the optical axis using a mechanical structure so as to adjust the image formation position mechanically. However, costs will be raised and the power consumption of the device will increase due to the extra parts required for the mechanical structure and a detector that detects the misplacement of the image formation position.
As described above, in the resin lens, the change in the curvature radius or the refractive index due to environmental temperature change is greater compared to a glass lens, and therefore, a field curvature is generated in the optical system that implements the resin lens and the beam spot diameter formed on the photoconductor is enlarged, resulting in image degradation. Various technologies for countering the above problem have been proposed in the prior art inventions; however, there have been no disclosures of an optical scanner device implementing a resin lens as the scanning lens that is capable of preventing the enlargement of a beam spot without requiring greater processing precision or mounting precision, and also without increasing costs by implementing additional mechanical parts or detection parts.